Thesis Proposal
Chris Crawford
November 14, 2001
Precision measurement of the proton electric to
magnetic form factor ratio with BLAST
Spokespersons: H. Gao, J.R. Calarco, H. Kolster
Outline
1. Introduction and Motivation
2. Existing Measurements
3. The Proposed Experiment
a. Overview
b. Formalism
c. Requirements
d. Measurement
4. Conclusions
1
1. Introduction and Motivation
 The electromagnetic form factors are fundamental
quantities related to the distribution of charge and
magnetization within a nucleon.
 The electromagnetic form factor ratio is also important,
as any Q2 dependence will suggest different charge and
magnetization spatial distributions inside the nucleon.
o High Q2 (beginning at 0.5 GeV2 ) results from JLAB
show a downward trend in this ratio for the proton.
o More recent higher Q2 data from JLAB show a
continuing downward trend up to Q2 =5.6 GeV2.
o Low Q2 data from 0 – 1 GeV2 is the focus of this
experiment, which is required to determine the
turning point in the Q2 dependence of the ratio.
2
Theory
 Lattice calculations:
o expected in the near future for low Q2 (0 – 1 GeV2).
 Models with explicit meson degrees of freedom:
o Vector Meson Dominance (VMD), Höhler 1976.
Photons couple to hadrons only via intermediate
vector bosons.
o VMD and Chiral Perturbation Theory (ChPT),
Mergell et al. 1996.
o Soliton Model, Holzworth 1996. This treats the
Baryon as an extended object with coupling to
vector mesons.
 QCD based Quark models:
o Constituent quarks, Frank et al. 1996. This model
constructs valence quark wavefunctions and does
not include the pion cloud or gluons.
o Cloudy bag, Lu et al. 1998. Photons couple with a
valence quark core and meson fields.
3
4
2. Measurement Techniques
 Rosenbluth separation
unpol
P2
P2



G E GM
 d 
1
2  e

P2
  Mott f recoil 
 2 GM tan 


2 

1


 d 


2
2
o Plot of d d vs. tan2(/2) gives GMP and GEP
o Difficult at Q 2  1 GeV 2 ( GEP ), and low Q 2 ( G MP ).
 
 Polarization transfer measurements p (e , ep ) using a
Focal Plane Polarimeter (FPP).
Pt E  E    e 
GEP


tan 
P
GM
Pl 2M P
2
Pt and Pl of the scattered proton were measured
simultaneously within the FPP using scattering from 12 C .
 
o Milbrath et al. (BATES), 1999. Also d (e , ep)n
o Jones et al. (JLAB), 2000.
o Dieterich et al. (MAMI, Mainz), 2001.
 Polarized beam and target measurement
o Well suited to BLAST
o Different systematics than above
o Insensitive to beam and target polariazation
o Our proposed experiment
5
Unpolarized Scattering and Polarization Transfer Data
Polarization Transfer Data
6
3. The Proposed Experiment
3a. Overview
 
 Will use the reaction p(e , ep) to measure  p GEP GMP
at Q2 = 0.07 – 0.9 GeV 2 to unprecedented precision at
beam energies 440 MeV and 880 MeV.
 Completely different experimental technique:
polarized H target instead of recoil proton polarimeter.
 Fits nicely between higher Q2 data at JLAB
and lower Q2 data from RPEX (future exp at BATES)
  p GEP GMP from Q2 = 0.02 – 6 (GeV/c) 2.
 Approved at the 28th BATES PAC with the highest
scientific priority.
7
3b. Formalism
 A longitudinally polarized electron scatters from a
polarized proton in the lab frame.
 The proton may be polarized in any direction described

*
*
by θ and φ with respect to q .
 The cross section for polarized scattering
d
   h
d
where h is the helicity of the electron beam,  is the
unpolarized cross section and  is the spin dependent
part.
8
 Theoretical asymmetry:
2 vT  cos * G MP  2 2 (1   )vTL sin  * cos  *G MP G EP

A 
2
2

(1   )v L G P  2 vT G P
2
E
M
 Experimental asymmetry:
Aexp
N  N
 Pb Pt A 
N  N
 Ratio of AL and AR (same Q 2 ):
Rexp
AL 2 vT  cos L*  2 2 (1   )vTL sin  L* cos L* GEP GMP


AR 2 vT  cos R*  2 2 (1   )vTL sin  R* cos R* GEP GMP
 We are therefore able to extract the form factor ratio
independent of the beam and target polarizations to
first order.
9
3c. Requirements
 Polarized electron beam stored in the ring (SHR)
o 60% polarization, 80μA average current
o Compton polarimeter
 Polarized internal hydrogen target
o Atomic Beam Source (ABS)
o Laser Driven Target (LDT)
 Symmetric Large Acceptance Detector (BLAST).
o Toroidal Magnetic Field
o Wire Chambers
o Timing Scintilators
o Cěrenkov
o Lead Glass Calorimeter
o Neutron Detectors
10
Polarized Beam
Asymmetry
 The Compton Polarimeter
0.01
0.008
0.006
0.004
0.002
-0
-0.002
-0.004
-0.006
-0.008
-0.01
2468
10
12
14
Gamma Energy (MeV)
11
Polarized Target
 Atomic Beam Source
o Well established technology
o Can create pure spin states
o Polarization:
o Flux:
o Thickness:
P = 0.8
 = 7×1016 /s
t = 5.5×1013 /cm2
 Laser Driven Target
o Compact design
o Active pumping—higher flux
o Flux:
 = 2×1018 atoms/s
o Atomic fraction F = 0.6 (expected)
o Polarization
P = 0.5 (expected)
12
Atomic Beam Source
13
Atomic Beam Source
14
Laser Driven Target
 Optical Pumping and Spin Exchange
rf
OPTICAL
PUMPING
K
K
H2
H
K
H
KH
SPIN EXCHANGE
OPTICAL
PUMPING
K
H
K
H
H
K
K
H
HH
SPIN EXCHANGE
H
H
H
 Schematic of Target
RF dissociator
H2
potassium ampoule
spincell
magnetic
holding field
circularly polarized laser
storage cell
15
16
 Dissociation Results:
Atomic fraction at the target cell using the new dissociator.
95
Atomic Fraction (%)
90
85
80
75
70
65
Spincell heating up
60
Potassium heating up, spincell at 180C
55
20
40
60
80
100
120
140
160
180
Temp (C)
 Polarization Results:
Negative Helicity
Positive Helicity
10
10
Laser transmission
9
9
8
8
7
7
6
6
5
5
4
4
3
Laser transmission
3
QMA mass 1 signal
2
2
1
1
QMA mass 1 signal
0
0
770.105
Laser wavelength (nm)
770.115
770.105
Laser wavelength (nm)
770.115
17
BLAST Detector
18
Monte Carlo
Momentum Reconstruction
h1
Theta Reconstruction
h3
Nent = 3990
Mean = -0.03706
RMS = 0.9919
240
220
Nent = 4142
Mean = 0.02022
RMS = 0.639
600
200
180
500
160
400
140
120
300
100
80
200
60
40
100
20
0
-5
-4
-3
-2
-1
012345
% error inp
0
-5
-4
-3
-2
-1
012345
% error in q
19
3d. Measurement
E(MeV)
440(c)
880(c)
880(s)
880(c)*
 e min(deg)  e max(deg)  p min(deg)  p max(deg)
33.5
26.5
82.5
82.5
103.0
113.0
37.8
30.5
18.9
18.9
66.2
65.5
30.5
22.3
(c) coincidence
(s) proton singles
*
(c) coincidence with extra detectors
 Beam time: 300 hrs (440 MeV), 900 hrs (880 MeV)
 Luminosity: L = 2.7×1031 s-1 cm-2
E = 440 MeV
<Q >
rate
(MeV2)
(Hz)
0.07
33.5
0.10
8.5
0.13
4.4
0.16
2.4
0.20
1.4
0.23
0.9
2
dR/R
(%)
0.46
0.69
1.47
1.??
1.70
1.72
E = 880 MeV
<Q >
Rate
(MeV2)
(Hz)
0.21
6.41
0.34
1.67
0.47
0.56
0.59
0.23
0.69
0.11
0.85
0.08
2
dR/R
(%)
0.40
0.35
0.44
0.66
1.17
2.49
 Systematic Errors
o scattered electron energy
o scattered electron angle
o target spin direction
20
21
4. Conclusions
 This experiment will be the first determination of the
 
proton form factor ratio using the p(e , ep) reaction.
 The proton form factor ratio will be measured from
Q2 = 0.07 to 0.9 (GeV/c) 2 to unprecedented precision
and dominated by statistical error.
 This experiment is well suited for BLAST.
 In the future a 1.1GeV beam may be used with the same
technique with BLAST to determine the proton form
factor ratio to Q2 = 1.2 (GeV/c) 2.
Acknowledgements
 B. Clasie, J. Seely, H. Kolster, V. Ziskin,
M. Farkhondeh, B. Franklin, T. Smith
22
References
Proposal to the MIT-Bates PAC “Precision measurement of
the proton electric to magnetic form factor ratio with BLAST”.
M. Jones et al., Phys. Rev. Lett. 84, 1398 (2000).
B. Milbrath et al., Phys. Rev. Lett. 80, 452 (1998), Phys.
Rev. Lett. 82, 221 (E) (1999).
S. Dieterich et al., Phys. Lett. B500, 47 (2001).
T. W. Donnelly et al., Ann. Phys. 169, 247 (1986).
G. Höhler et al., Nucl. Phys. B114, 505 (1976).
P. Mergell et al., Nucl. Phys. A596, 367 (1996).
G. Holzworth, Z. Phys. A356, 339 (1996).
M. R. Frank et al., Phys. Rev. C54, 920 (1996).
D. H. Lu et al., Phys. Rev. C57, 2628 (1998).
23